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Report on Aira (Japan) — August 1995


Bulletin of the Global Volcanism Network, vol. 20, no. 8 (August 1995)
Managing Editor: Richard Wunderman.

Aira (Japan) Explosive activity 23-25 August, dense ash cloud closes a highway

Please cite this report as:

Global Volcanism Program, 1995. Report on Aira (Japan) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 20:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199508-282080



31.593°N, 130.657°E; summit elev. 1117 m

All times are local (unless otherwise noted)

During 1995, the geophysical system described below registered Sakura-jima's 126th explosion on 23 August. During 23-25 August, 28 explosions were recorded. The total through August of 153 explosions is relatively small compared to 1960, 1974, 1983, and 1985, years when over 400 explosions took place. During August no measurable ash fell at Kagoshima Local Meteorological Observatory, 10 km W of the crater. On the other hand, ash-bearing explosions were of sufficient size to send a dense ash cloud NW of the volcano that dropped ash in N Kyushu and closed a highway for an unspecified duration. The highest plume of the month vented on 30 August and rose to 3 km above the crater rim. Station B (2.3 km NE of Minami-dake crater) registered 671 earthquakes and 378 tremor events.

Geophysical determination of explosions. The monthly tally of "explosions" (sometimes also called "explosive eruptions") at Sakura-jima has a geophysical definition, with its origins closely linked to aircraft safety. The volcano sits ~25 km from the busy Kagoshima International Airport and generates frequent Vulcanian explosive eruptions (BGVN 19:11). A video camera monitors the volcano and a real-time image is transmitted to air traffic control. In order to alert aviation dispatchers and pilots of potential hazards regardless of the time of day or the weather, scientists devised a system to rapidly classify the volcano's seismic and acoustic signals (Onodera and Kamo, 1994). This geophysical system has been linked to the Japan Airlines office at Kagoshima Airport since March 1991.

When the amplitudes of incoming seismic signals rise above an established threshold (table 12) their dominant frequency is computed. The above-threshold signals also have an associated air-shock wave that is received at an "infrasonic" microphone with a 0.02-100 Hz detection range. For reference, the low-frequency range of the human ear stops at around 16 Hz. Once the infrasonic air-shock wave is received, the system measures its amplitude and computes its spectrum. The combination of seismic and air-shock amplitudes and spectra allow the events to be classified into "non-eruption," "eruption," or "explosion." categories (table 2).

Table 12. (above) Definition of "explosion" at Sakura-jima, a geophysical characterization of explosive, bomb- and ash-bearing eruptions. (below) Recognized volcanic earthquakes at Sakura-jima, showing the maximum values registered for explosion earthquakes and corresponding air-shock waves (after Onodera and Kamo, 1994).

Criteria for Definition of Explosion at Sakura-jima (~90% accurate when compared to visual observations)
1. Maximum amplitude of explosion earthquake >= 10 microns (0.1 x 10-3 cm/sec).
2. Amplitude of infrasonic air shock >= 0.1 mbars at a site 2.7 km NW of summit crater.
3. Spectral analysis of received infrasonic air-shock discriminates between the categories "non-eruption" (> 5 Hz), "eruption" (2- 5 Hz), and "explosion" (<2 Hz).
Volcanic earthquake type Dominant frequency range Comment
A-type >8 Hz Similar to tectonic earthquakes; devoid of infrasonic air-shocks and not accompanied by eruptive activity.
B-type <5 Hz Includes both BL (1-3 Hz, max. amplitudes <7 x 10-3 cm/sec, reduced displacement <60 cm2 and <1 mb) and BH (5-8 Hz); the former often affiliated with bomb- and ash-bearing eruptions; the latter not affiliated with eruptive activity.
C-type -- Harmonic wave trains, "volcanic tremor"
D'-type -- Non-harmonic tremor; max. amplitudes <7 x 10-3 cm/sec, reduced displacement <60 cm2 and <1 mb; often affiliated with bomb- and ash-bearing eruptions.
Explosion -- Accompanied by strong air-shock waves, and bomb- and ash-bearing. Maximum amplitudes range from 3 x 10-3 to 3 x 10-2 cm/s for the earthquakes (reduced displacements of 50-500 cm2) and 0.1 to 5 mb for the infrasonic air-shock waves.

Although passing typhoons can trigger inappropriate warnings or false alarms, and small-magnitude eruptions may be missed, the number of explosions correlates well with the measured deposition of fresh volcanic ash. The system has been effective at reducing aviation risks. A future goal is to use "explosion" category to automatically trigger the calculation of volcanic ash diffusion based on meteorological data. This program would thus automatically estimate the likely trajectory of ash discharged from the volcano.

References. Onodera, S., Iguchi, M., and Ishihara, K., 1994, Recent advances in Japan, volcano monitoring system of Japan Airlines at Kagoshima Airport: 9th Annual International Oceanic Airspace Conference, 9 November 1994.

Geological Summary. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu's largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical eruption took place during 1471-76.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan; Saburo Onodera, Director of Meteorology, Flight Operations, Japan Airlines, 3-3-2 Haneda Airport, Tokyo 144, Japan; Kosuke Kamo, Masato Iguchi, and Kazuhiro Ishihara, Sakurajima Volcano Observatory (SVO), Disaster Prevention Research Institute, Kyoto University, Sakurajima-cho, Kagoshima 89114, Japan.